1. Field of the Invention
This invention relates to an apparatus and a method for encoding a picture by wavelet transform and entropy coding, for compression, as for example in JPEG-2000 system, a program for having a computer execute picture encoding processing, and a recording medium having the program recorded thereon.
2. Description of Related Art
Among known typical picture compression systems, there is a JPEG (Joint Photographic Experts Group) system, standardized by ISO (International Standards Organization). This system uses discrete cosine transform and is able to provide satisfactory encoded and decoded pictures when a larger amount of bits are allocated. However, if the number of bits for encoding is decreased to more than a certain extent, there occurs pronounced block distortion peculiar to DCT. The result is the pronounced subjective deterioration.
On the other hand, research in systems for splitting the frequency spectrum of a picture into plural frequency bands by a filter composed of a high-pass filter and a low-pass filter, known as a filter bank, and for performing encoding from one such frequency band to another, are progressing briskly. Of these systems, these systems, wavelet transform encoding is free from the drawback that block distortion becomes severe with higher compression, as in the case of the DCT, and hence is felt to be promising as a new technology which should take the place of the DCT.
The JPEG-2000 system, the international standardization for which has been completed in January 2001, uses a system which combines this wavelet transform with the high efficiency entropy encoding (bitplane based bit modeling and arithmetic coding) and has achieved marked improvement in the encoding efficiency as compared to JPEG.
These international standards provide only for decoder side regulations, such that the side encoder is able to attempt the designing freely. However, there lacks the standard relevant to an effective rate controlling method for achieving the target compression ratio, and hence the needs for setting up a proper knowhow are incumbent.
In particular, in the JPEG system, this rate control is difficult with the consequence that encoding needs to be performed a plural number of times until the target value is achieved. However, this leads to increased processing time. Thus, in the JPEG-2000 system, it is desired to achieve the target code volume by encoding only once.
In the JPEG-2000 system, the rate controlling method exploiting RD (rate distortion) characteristics is used by and large. However, this rate controlling method, while exhibiting all-purpose properties, suffers from an extremely high calculating load.
The present Assignee has already proposed in the undermentioned Cited Reference 1 a technique in which an encoded code stream, once generated, is truncated from the trailing end to perform rate control. With this technique, control correctly matched to the target code volume may be achieved.
Cited Reference 1
Japanese Laying-Open Patent Publication 2002-165098
However, if, with the technology of the Cited Reference 1, a code block which affects the picture quality appreciably is at the trailing end of the encoded code stream, the picture quality tends to be deteriorated by truncating the code block. Thus, a more effective rate control technique has been desired.
Moreover, there lacks in the JPEG-2000 system the standard relevant to the technique of relieving the usually high processing load of entropy encoding and hence the needs for setting up a proper know how are incumbent.
Additionally, the JPEG-2000 system is the standard for still pictures and no sufficient measures have taken for interlaced pictures which occur frequently in the moving pictures. Thus, if the technique used for still pictures is directly used for moving pictures, there occurs a noticeably deteriorated picture quality.